This invention is directed to the partial or entire replacement of rhodium as a catalyst in a catalyst system, such as a three way catalyst (TWC) system which can be advantageously used for automotive emission control. The catalysis of interest is for the conversion of hydrocarbons, nitrogen oxides, and carbon monoxide into their nontoxic gaseous counterparts. The catalyst system of the present invention involves mechanisms wherein intermediate products of the initially catalyzed reactants may be further catalyzed to form carbon dioxide, nitrogen gas, and water.
The conversion of noxious exhaust fumes is a problem that has long followed the trail of the automotive industry. In recent times efforts have been expended to decrease the extent of toxic gases emitted from exhaust systems through the use of catalytic reactions. A problem long-felt in the industry had been the need for a catalytic system which was able to cope with the potpourri of reduced and oxidized forms of carbon and nitrogen. To this end, TWC catalyst systems have evolved to enable conversion of the reaction products of internal combustion engines.
A common problem to other organically fueled energy conversions, such as coal or oil fired power plants, is the production of the same kinds of pollutants found in the automotive industry. Therefore, a system that can be adapted to automotive exhaust gases can fittingly be adapted to industrial smoke stacks or organically fueled power sources.
One component in the TWC catalyst system that has been found effective for the catalytic reduction of nitrogen oxides is rhodium. Unfortunately, rhodium is a rare and expensive resource. Catalyst systems have been researched, therefore, in an effort to replace or substitute for rhodium. "The Use of Molybdenum in Automotive Three-Way Catalysts", by Gandhi et al, printed in ACS Symposium Series, No. 178, 1982, discusses the benefits of substituting molybdenum for some of the rhodium in a TWC catalyst system. The obvious benefit to a decreased use of rhodium is the significant decrease in the price of the catalyst system. A substitute for rhodium, therefore, is of paramount importance to some very large industries.
U.S. Pat. No. 4,678,770 discloses the use of platinum and/or palladium, rare earth oxides, alumina, and rhodium. This combination is used as a TWC catalyst system.
U.S. Pat. No. 4,297,328 discloses the use of copper, copper ions, and zeolites in a TWC catalyst system. Platinum and palladium are not used in this system.
The present invention substitutes therefor and/or replaces partially or entirely rhodium in a catalyst system, maintaining in the system palladium and/or platinum. It has been discovered through the use of molecular sieves, such as zeolites, and, singly or a combination of, platinum and/or palladium, that this combination of catalysts performs equally well as the rhodium/platinum and/or palladium metal system.